Trivalent Actinide Ions Showing Tenfold Coordination
in Solution
Posted on 2020-08-14 - 16:34
Trivalent actinides generally exhibit
ninefold coordination in
solution. 2,6-Bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP),
a tridentate nitrogen donor ligand, is known to form ninefold coordinated
1:3 complexes, [An(nPr-BTP)3]3+ (An = U, Pu,
Am, Cm) in solution. We report a Cm(III) complex with tenfold coordination
in solution, [Cm(nPr-BTP)3(NO3)]2+. This species was identified using time-resolved laser fluorescence
spectroscopy (TRLFS), vibronic side band spectroscopy (VSBS), X-ray
photoelectron spectroscopy (XPS), and density functional theory (DFT).
Adding nitrate to a solution of the [Cm(nPr-BTP)3]3+ complex in 2-propanol shifts
the Cm(III) emission band from 613.1 to 617.3 nm. This bathochromic
shift is due to a higher coordination number of the Cm(III) ion in
solution, in agreement with the formation of the [Cm(nPr-BTP)3(NO3)]2+ complex. The formation of this
complex exhibits slow kinetics in the range of 5 to 12 days, depending
on the water content of the solvent. Formation of a complex [Cm(nPr-BTP)3(X)]2+ was not
observed for anions other than nitrate (X– = NO2–, CN–, or OTf–). The formation of the [Cm(nPr-BTP)3(NO3)]2+ complex was studied as a function
of NO3– and nPr-BTP concentrations, and
slope analyses confirmed the addition of one nitrate anion to the
[Cm(nPr-BTP)3]3+ complex. Experiments with varied
nPr-BTP concentration show that [Cm(nPr-BTP)3(NO3)]2+ only forms at nPr-BTP
concentrations below 10–4 mol/L whereas for concentrations
greater than 10–4 mol/L the formation of the tenfold
species is suppressed and [Cm(nPr-BTP)3]3+ is
the only species present. The presence of the tenfold coordinated
complex is supported by VSBS, XPS, and DFT calculations. The vibronic
side band of the [Cm(nPr-BTP)3(NO3)]2+ complex exhibits a nitrate stretching
mode not observed in the [Cm(nPr-BTP)3]3+ complex.
Moreover, XPS on [M(nPr-BTP)3(NO3)](NO3)2 (M = Eu, Am) yields
signals from both non-coordinated and coordinated nitrate. Finally,
DFT calculations reveal that the energetically most favored structure
is obtained if the nitrate is positioned on the C2 axis of the D3 symmetrical
[Cm(nPr-BTP)3]3+ complex with a bond distance
of 413 pm. Combining results from TRLFS, VSBS, XPS, and DFT provides
sound evidence for a unique tenfold coordinated Cm(III) complex in
solutiona novelty in An(III) solution chemistry.
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Weßling, Patrik; Schenk, Tobias; Braun, Felix; Beele, Björn B.; Trumm, Sascha; Trumm, Michael; et al. (2020). Trivalent Actinide Ions Showing Tenfold Coordination
in Solution. ACS Publications. Collection. https://doi.org/10.1021/acs.inorgchem.0c01526
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AUTHORS (10)
PW
Patrik Weßling
TS
Tobias Schenk
FB
Felix Braun
BB
Björn B. Beele
ST
Sascha Trumm
MT
Michael Trumm
BS
Bernd Schimmelpfennig
DS
Dieter Schild
AG
Andreas Geist
PP
Petra J. Panak